Proof that any person, firm, company, association or corporation has been discriminating between different sections, communities and cities of this state by selling a commodity at a lower rate in one section, community or city than is charged for said commodity by said party in another section, community or city, after making an allowance for the difference, if any, in the grade or quality and in the actual cost of transportation from the point of production, if a raw material, and from the point of manufacture, if a manufactured product, shall be prima facie evidence that the party so discriminating is guilty of unfair discrimination.